Newbie with questions
Hi everyone,
I'm very glad to be here for the first time.
Thank you John for manually registered me in.
Thank you Tom Van Baak for putting my e-mails here and also
for your great answer which clear up most of my confusions.
Sorry for my poor English because its not my mother tongue.
I come from a place where there are few people who are
interested in time and frequency so I learn most of the
subject by myself mainly. Until one day I came across an
article by Tom and then his leapsecond.com which I read
with great interest, then I found this mailing list. After
read most of the archives I understand I belongs to here.
I have a lot of questions now (and probably more in the future).
Here is my first set of questions.
We all know that Allan Variance/Deviation (ADEV) is one of the
common ways to measure time domain noise of frequency sources.
It seems to me that the noise of many time-interval equipment
can also be described in this way.
Q1A: Is ADEV could be used for all time-interval measuring
equipments?
An example is at: http://www.timing.com/products/testsets.php
it says ADEV <5x10-14 at 1 second for TSC 5110A
If so, or partly so,
Q1B: Could it be easily measured/confirmed in general?
I even have difficulties measuring the ADEV of SR620 which has
the ADEV function build in. I made wrong assumption in my last
mail to Tom, so I re-measure it below.
Q1C: I actually measured the ADEV of my frequency counter
SRS SR620 below (for tau = 1 sec), anything wrong?
SR620 is able to take many sample frequencies and calculate the
ADEV and standard deviation (SDEV). What happened if it measure
the reference frequency?
Using a good 10MHz as a external reference for SR620, also
Tee this frequency and let the counter measure it at the same
time. Ideally it will read 10MHz exact. However, because of
the jitter (and some other reason) of the counter, the reading
is not perfect. Set the gate time to 1 second, number of
samples to 100, the reading for Jitter:Allan is as follows:
the counter at tau=1 second.
1.5E-11 for HP Z3801A (no GPS)
3.7E-11 for SR625 (Rubidium time base)
1.6E-11 for HP 58540A (GPS locked)
1.3E-11 for HP 58540A (Holdover)
1.1E-11 for Trimble Thunderbolt (GPS locked, time const=400s)
1.1E-11 for Trimble Thunderbolt (Holdover)
1.4E-11 for SR620 its own internal reference (TCXO)
(during above measurement, any reading are taken for at least
three times, I keep steady, and no cables are touched)
It is easy to say that:
- the lower the reading, the better the frequency source for
tau = 1sec
-
compact Rubidium is worse than OCXO in short time stability.
-
the ADEV of the SR620 (if it applies) is better
than any of the above readings
- the ADEV of the SR620 is perhaps not too much
better than the lowest reading of above, 1.0E-11 is my estimate.
- this 1E-11 is equivalent to 10ps jitter which is good
because SR620 has 100ps uncertainty for frequency measurement
as specified.
- SR620 cannot measure ADEV of much lower than 1E-11 for
tau = 1 sec. because of its own noise floor. This makes it
useless to measure ADEV of good OCXOs.
Q1D: Is the ADEV and the SDEV always the same for frequency
counter noise?
I ask this because on my SR620, it almost the same reading
for Jitter:Allan and Jitter:STD DEV regardless of the gate
time and the frequency source. Tom has answered this question
by saying that it will be the same in his previous mail.
Q1E: which is better as for noise, Agilent 53131A or SR620?
For Agilent 53131A, only the SDEV function is available.
I made the similar measurement as above (gate time=1s, N=100
samples). The SDEV is worse than those obtained by SR620.
For instance:
2.3E-11 for HP Z3801A (no GPS)
2.1E-11 for HP 58540A (GPS locked)
2.3E-11 for Trimble Thunderbolt (GPS locked, time const=400s)
2.4E-11 for SR625 (Rubidium time base)
3.1E-11 for 53131A its own internal reference (TCXO)
Yes, these readings are quite the same for external references.
It seem to me that the 53131A uses external reference directly
while SR620 uses the external frequency indirectly by phase
locking an internal VCXO to it. This 2.3E-11 deviation is
equivalent to 23ps of jitter which is very good comparing to
the 500ps error spec. for frequency measurement.
correct me if I am wrong.
Thank you for your attention.
Lymex, BG2VO
---=
I believe the SRS 620 is a better instrument than the HP 53131A. The
53131A has a single shoot resolution rated at 500 picoseconds. From
test I have run, it can vary 190 pS to +500 pS.
I am in the process of running variance test on my disciplined rubidium,
and I baseline the counter and then compare against several references.
The baseline consist of of connecting a 1 PPS reference to the time
interval counter stop and start input, from the same timebase to the
counter, and then using 1 PPS from 3 other references. The measurements
presented are 4 samples/sets averaged together (the actual samples of
the datasets are much, much higher).
As a quick reference of my 53131A, the noise floor at 1 second is
1.7x10-10, 3 seconds is 4.0x10-11, for 6 seconds is 1.4x10-11, 10
seconds is 4.8x10-12, 30 seconds is 1.3x10-12, 60 seconds is 7.4x10-13.
This test is still in progress, so I do not have data for higher
averages (yet).
I believe this is something we should all do, to get to know our
instruments. Its a way to validate the data, considering that most of
us do not have our test equipment in a calibration program.
Brian - N4FMN
lymex@vip.sina.com wrote:
Q1E: which is better as for noise, Agilent 53131A or SR620?
For Agilent 53131A, only the SDEV function is available.
I made the similar measurement as above (gate time=1s, N=100
samples). The SDEV is worse than those obtained by SR620.
Why we are talking about resolution and noise floor, I am looking for a
merit figure to say the data I have measured, is valid.
As an example, when I worked in satellite communications, we had a
measurement called C/kt. This is a carrier to noise measurement,
normalized back to a 1 hertz bandwidth. When we made these measurements
with a spectrum analyzer, we had to account for the measurement
bandwidth of the instrument, the detector used (factors for sine wave
response vs. gaussian noise), etc. Basically if the reading was 6 db
above the noise floor, then we could believe the measurement.
In my case, were I have baselined the HP53131A noise floor, I know my
readings have to be above this floor, to be valid.
My question is how much should our readings be above the noise floor to
be considered valid, and what units/formulas should we use ? Can we
convert Allan variances to a decibel reading, and use similar techniques
as I described?
Brian – N4FMN
Hi everyone,
I'm very glad to be here for the first time.
Thank you John for manually registered me in.
Thank you Tom Van Baak for putting my e-mails here and also
for your great answer which clear up most of my confusions.
Sorry for my poor English because its not my mother tongue.
I come from a place where there are few people who are
interested in time and frequency so I learn most of the
subject by myself mainly. Until one day I came across an
article by Tom and then his leapsecond.com which I read
with great interest, then I found this mailing list. After
read most of the archives I understand I belongs to here.
I have a lot of questions now (and probably more in the future).
Here is my first set of questions.
We all know that Allan Variance/Deviation (ADEV) is one of the
common ways to measure time domain noise of frequency sources.
It seems to me that the noise of many time-interval equipment
can also be described in this way.
Q1A: Is ADEV could be used for all time-interval measuring
equipments?
An example is at: http://www.timing.com/products/testsets.php
it says ADEV <5x10-14 at 1 second for TSC 5110A
Yes, that is the noise floor specification for the
TSC 5110A - the minimum resolution that it can
measure. Every oscillator has a certain specified,
or measured, ADEV plot; the same is true for any
test equipment that does timing measurements.
The resolution specification is not always listed
as ADEV. Some counters advertise the "number
of digits per second", or the single-shot resolution,
or the LSD (least significant digit). But it's all
essentially the same thing.
If so, or partly so,
Q1B: Could it be easily measured/confirmed in general?
I even have difficulties measuring the ADEV of SR620 which has
the ADEV function build in. I made wrong assumption in my last
mail to Tom, so I re-measure it below.
You can indirectly measure the ADEV of the
test equipment if you know the ADEV of each
of the two oscillators you are comparing with
that piece of test equipment. If, for example,
you have two nice OCXO that you know have
an ADEV of, say, 1e-12 at 10 seconds and
your counter tells you their relative ADEV is
1e-11, then you can be sure it's the counter
that is the limiting factor, not either of the
sources.
In general, the ADEV you measure is the RMS
of the stability of each source and the TI counter
itself. And so in the special case that the OCXO
perform identically and the TI counter is much
better than the OCXO the true ADEV of each
of the OCXO is sqrt(2) of the measured ADEV.
Does this make sense?
Q1C: I actually measured the ADEV of my frequency counter
SRS SR620 below (for tau = 1 sec), anything wrong?
SR620 is able to take many sample frequencies and calculate the
ADEV and standard deviation (SDEV). What happened if it measure
the reference frequency?
This is a sort of self-test and it gives you a
rough measure of the jitter in the system.
Using a good 10MHz as a external reference for SR620, also
Tee this frequency and let the counter measure it at the same
time. Ideally it will read 10MHz exact. However, because of
the jitter (and some other reason) of the counter, the reading
is not perfect. Set the gate time to 1 second, number of
samples to 100, the reading for Jitter:Allan is as follows:
the counter at tau=1 second.
1.5E-11 for HP Z3801A (no GPS)
3.7E-11 for SR625 (Rubidium time base)
1.6E-11 for HP 58540A (GPS locked)
1.3E-11 for HP 58540A (Holdover)
1.1E-11 for Trimble Thunderbolt (GPS locked, time const=400s)
1.1E-11 for Trimble Thunderbolt (Holdover)
1.4E-11 for SR620 its own internal reference (TCXO)
(during above measurement, any reading are taken for at least
three times, I keep steady, and no cables are touched)
This is a nice set of data.
It is easy to say that:
- the lower the reading, the better the frequency source for
tau = 1sec
Correct.
- compact Rubidium is worse than OCXO in short time stability.
Yes, this is often true. Many OCXO have much
better short-term stability than compact Rb.
- the ADEV of the SR620 (if it applies) is better
than any of the above readings
I'm not sure you can say this. It might be
equal to some of the readings.
I would guess that one or more of your sources
is much better than 1e-11. The fact that all the
readings are above 1e-11 suggests that the
SR620 itself is the limiting factor here. And,
based on my earlier email, this is to be
expected -- 1e-11 is 10 ps per second. I can't
see the SR620 getting much better than this
- the ADEV of the SR620 is perhaps not too much
better than the lowest reading of above, 1.0E-11 is my estimate.
Correct.
- this 1E-11 is equivalent to 10ps jitter which is good
because SR620 has 100ps uncertainty for frequency measurement
as specified.
- SR620 cannot measure ADEV of much lower than 1E-11 for
tau = 1 sec. because of its own noise floor. This makes it
useless to measure ADEV of good OCXOs.
Yes and no. It is useless to measure ADEV of
OCXO for short tau. It should still be very useful
for longer tau. The resolution is approx 1e-11 / tau
so at 100 seconds the SR620 can produce data
that gives you ADEV down to 1e-13.
It is not uncommon to use multiple methods
to achieve a full ADEV plot, from sub-second
to multiple days or weeks. A single reference
or a single counter is just not good enough to
cover the entire spectrum. Some people, for
example, use their best OCXO for tau 0.1 s
to tau 500 s, and use GPS for tau greater than
about 1000 s.
The SR620 is one of the best, but it does have
its limits, as does every instrument.
Measuring ADEV of good OCXO (1e-12 to 1e-13)
is not that hard; but you have to use methods
other than simple frequency or TI counters. That's
why I mentioned the heterodyne (DBM) method.
Q1D: Is the ADEV and the SDEV always the same for frequency
counter noise?
Mostly this is true. When dealing with white
noise the two will be similar. Where ADEV
and stddev diverge is when there are drift
effects; this is usually something that happens
in the mid- to long-term, not short term.
If you ran a TI counter for a day you would
definitely see periodic, drift, or thermal effects
in the data. stddev sweeps all this data into
one calculation while ADEV(tau) can resolve
different parts of the time spectrum.
I ask this because on my SR620, it almost the same reading
for Jitter:Allan and Jitter:STD DEV regardless of the gate
time and the frequency source. Tom has answered this question
by saying that it will be the same in his previous mail.
For white noise this will be the case.
With the SR620, you will start to see Jitter:Allan
and Jitter:stddev differ for longer runs.
Q1E: which is better as for noise, Agilent 53131A or SR620?
For Agilent 53131A, only the SDEV function is available.
I made the similar measurement as above (gate time=1s, N=100
samples). The SDEV is worse than those obtained by SR620.
For instance:
2.3E-11 for HP Z3801A (no GPS)
2.1E-11 for HP 58540A (GPS locked)
2.3E-11 for Trimble Thunderbolt (GPS locked, time const=400s)
2.4E-11 for SR625 (Rubidium time base)
3.1E-11 for 53131A its own internal reference (TCXO)
The SR620 should be much better than a
53131A, or even a 53132A. It will be about
the same as a 5370.
Yes, these readings are quite the same for external references.
It seem to me that the 53131A uses external reference directly
while SR620 uses the external frequency indirectly by phase
locking an internal VCXO to it. This 2.3E-11 deviation is
equivalent to 23ps of jitter which is very good comparing to
the 500ps error spec. for frequency measurement.
Not sure on this one. It could be that the time
constant of each PLL is different, or the TCXO
in each counter is of different quality.
correct me if I am wrong.
Thank you for your attention.
Lymex, BG2VO
/tvb
From: Brian Kirby kirbybq@bellsouth.net
Subject: [time-nuts] TI Counter Noise Floor/Resolution
Date: Thu, 28 Jul 2005 15:06:50 -0500
Message-ID: 42E93ADA.7050205@bellsouth.net
Brian,
Just back from a 4 day of music festival, so lagging behind somewhat.
Why we are talking about resolution and noise floor, I am looking for a
merit figure to say the data I have measured, is valid.
As an example, when I worked in satellite communications, we had a
measurement called C/kt. This is a carrier to noise measurement,
normalized back to a 1 hertz bandwidth. When we made these measurements
with a spectrum analyzer, we had to account for the measurement
bandwidth of the instrument, the detector used (factors for sine wave
response vs. gaussian noise), etc. Basically if the reading was 6 db
above the noise floor, then we could believe the measurement.
In my case, were I have baselined the HP53131A noise floor, I know my
readings have to be above this floor, to be valid.
My question is how much should our readings be above the noise floor to
be considered valid, and what units/formulas should we use ? Can we
convert Allan variances to a decibel reading, and use similar techniques
as I described?
Interesting view on things. A 10log(AVAR(tau)) would be the appropriate thing
to use, this would be equalent to a 20log(ADEV(tau)) since ADEV(tau) =
SQRT(AVAR(tau)).
Again, a 6 dB margin is would render the right neighborhood, but then again,
that is a rule-of-thumb rather than exact science.
Cheers,
Magnus